In many types of silver halide photographic element it is necessary to provide one or more dye layers separate from the emulsion layer(s), e.g. for filtering, antihalation or anticrossover purposes. In most cases it is essential that the dyes are bleached or washed out completely by processing solutions, so that there is no residual stain in the final image. However, it is equally essential that the dyes do not migrate from their intended layer(s) into adjacent emulsion layer(s) during coating or storage of the photographic elements, as this would lead to desensitization of the emulsion(s). Solid particle dye dispersions, and in particular, solid particle dispersions of dyes which are soluble under alkaline pH conditions, but insoluble under neutral or acidic pH conditions, provide an attractive solution to this problem. In such dispersions, the dyes exist as discrete solid particles (typically of the order of 1 mm in size) under neutral or acidic pH conditions, but dissolve in aqueous alkali. Hence, the dyes are in the form of solid particles under normal coating and storage conditions, and cannot migrate from their intended layer, but are readily dissolved out by typical alkaline photographic processing solutions.
A wide variety of dyes have been used in this way, as disclosed for example in U.S. Pat. Nos. 4,092,168; 4,288,534; 4,803,150; 4,900,652; 4,855,221; 4,940,654; 4,857,446; 4,861,700; 5,238,798; 5,238,799; 5,342,744; 5,356,766; EP-A-0594973 and 0694590. In most cases, alkaline solubility of the dyes is ensured by the presence of one or more carboxylic acid substituents. The solid particle dye dispersions may be formed by precipitation techniques, e.g. by controlled acidification of an alkaline solution of the relevant dye, as described in U.S. Pat. No. 3,560,214, EP-A-0724191 and U.S. Pat. No. 5,326,687, but are most commonly formed by grinding or milling the solid dye to the desired particle size in an aqueous medium, then mixing with gelatin or other hydrophillic colloid. In order to achieve a stable dispersion of suitably small particle size which is not prone to settling, aggregation, coagulation or other undesirable changes during storage, it is normal practice to add one or more surfactants or stabilizers before or after the milling process. For example, EP-A-0694590 discloses the use of a poly(ethyleneoxide)/poly(propylene oxide) block copolymer for this purpose and U.S. Pat. No. 5,300,394 discloses the use of a fluorosurfactant.
U.S. Pat. Nos. 5,468,598, 5,478,705, 5,500,331 and 5,513,803 disclose methods and materials relevant to the production of solid particle dispersions for use in imaging media, and provide lists of suitable surfactants. The surfactant/dispersing aid disclosed in the majority of the Examples in these and other prior art patents is an anionic surfactant called Triton X-200 which is supplied by Union Carbide.
Ideally, a surfactant/dispersing aid used for the preparation of solid particle dye dispersions for photographic use should be cheap, readily available, non-toxic, non-polluting, photographically inert, non-foaming, and should expedite the milling process as well as stabilizing the resultant dispersion. None of the materials disclosed in the prior art fulfills all these criteria, and in particular Triton X-200, the most commonly used material, is found to generate excessive amounts of foam during the milling process, and/or requires long milling times. (Milling times of several days are mentioned in the prior art.) Foaming is caused by entrapment of air during the milling process, and, generally speaking, the degree of foaming increases as the milling process becomes more vigorous. The presence of foam reduces the efficiency of the milling, and may prevent attainment of the desired particle size. If the foam is stable, i.e., does not collapse on standing for a prolonged period, the resulting dispersion may be unusable. In theory, the milling time to achieve a given particle size may be reduced by using a more vigorous milling process, but if foaming is induced or exacerbated, the exercise will be self defeating.
A related problem caused by air entrapment is that of bubble formation. Air may become trapped within the system in the form of bubbles dispersed throughout the liquid medium. If these remain stable after milling has ceased, the resulting dispersion clearly cannot be used for coating purposes, especially thin coatings. The bubbles cause voids and streaks in the coatings. Many conventional surfactants are found to give rise to this problem.
There is, therefore, a need for alternative dispersing aids for use in the production of solid particle dye dispersions for photographic use.
Glycerophospholipids, e.g., lecithin, are well-known dispersing and emulsifying agents, particularly in the food, cosmetic and pharmaceutical industries (see, for example, Kirk-Othmer Encyclopedia of Chemical Technology (4th edition), Vol. 15, pp. 192-210). Lecithin also finds use in magnetic recording media as a stabilizer for dispersions of metal oxide particles in hydrophobic organic binders, and as a pigment dispersant in water-based paints, but has not been widely used in photographic media.
U.S. Pat. No. 5,385,819 discloses the use of lecithin in the growth of tabular silver halide grains. JP55-088045 discloses the use of lecithin in the dispersion, in gelatin, of an oil containing a dye precursor.
DE 2,259,566 discloses the use of lecithin to stabilize a dispersion of silica particles in a photographic layer for antistatic or antifriction properties. The silica particles are formed in or reduced to the required particle size prior to mixing with the lecithin. DD 203,161 discloses the use of a lecithin derivative to stabilize a dispersion of carbon black in a phenolic resin binder, the formulation being used as an antihalation backcoat for a photographic element. The dispersion is formed in a non-aqueous system.